Toward Understanding the B[e] Phenomenon: V. Nature and Spectral Variations of the MWC 728 Binary System.

We report the results of a long-term spectroscopic monitoring of the FS CMa type object MWC 728. We found that it is a binary system with a B5 Ve (T-eff = 14,000 +/- 1000 K) primary and a G8 III type (T-eff +/- 5000 K) secondary. Absorption line positions of the secondary vary, with a semi-amplitude of similar to 20 km s(-1) and a period of 27.5 days. The system's mass function is 2.3 x 10(-2) M-circle dot, and its orbital plane is similar to 13 degrees-15 degrees tilted from the plane of the sky. The primary's upsilon sin i similar to 110 km s(-1), combined with this tilt, implies that it rotates at a nearly breakup velocity. We detected strong variations of the Balmer and He I emission-line profiles on timescales from days to years. This points to a variable stellar wind of the primary in addition to the presence of a circum-primary gaseous disk. The strength of the absorption-line spectrum, along with the optical and near infrared (IR) continuum, suggest that the primary contributes similar to 60% of the V-band flux, the disk contributes similar to 30%, and the secondary contributes similar to 10%. The system parameters, along with the interstellar extinction, suggest a distance of similar to 1 kpc, that the secondary does not fill its Roche lobe, and that the companions' mass ratio is q similar to 0.5. Overall, the observed spectral variability and the presence of a strong IR-excess are in agreement with a model of a close binary system that has undergone a non-conservative mass-transfer.